This invention relates to an apparatus and method for producing waterproof cable, particularly that type of cable such as communication cable made up of a great number of individual insulated wires bundled together in a tightly packed core. More specifically the invention deals with a method and apparatus for the application of vibrational energy to such jelly-like waterproofing compound so as to enable such to more rapidly and more completely penetrate the inner interstices of such core.
It is desirable to waterproof many types of cable, particularly communication cable, such as above ground or buried telephone communication cable, wherein a plurality of individual wire conductors are coated by extrusion with an insulating material, and each such insulated conductor is subsequently twisted with another insulated conductor to form a twisted pair. A plurality of such twisted pairs of insulated conductors are then stranded together to form a cable core. It is common that several hundred such conductors may be present in such cable cores.
Inasmuch as it is desirable to minimize, to the extent possible, the entrance of moisture into such cable core, it is common to attempt to fill the voids of the interstitial structure of the core of the cable with a compound which has properties sufficient to minimize or prevent the entry of such moisture. Generally such compounds should be capable of becoming at least semi-liquid at elevated application temperatures and pressures and capable of becoming viscous or solid state at those temperatures in which they are utilized in the finished cable structure and include mixtures of petroleum jelly, microcrystalline petroleum waxes, polyethylene, polyisobutylene, aluminum stearate, mixtures of whiting and castor oil, microcrystalline petroleum waxes, and blends of two or more such above-indicated filling materials. Such materials are generally referred to in the trade as jelly and are applied to the cable core in an elongated tube or chamber commonly referred to as a jelly tube. Accordingly, it is common to supply such jelly material in a semi-liquid or jelly-like state into the confines of such tube under a differential pressure caused either by the direct application of pressure to the material within the tube or by maintaining both the entrance and exit portions of the tube under evacuation. The cable core thus becomes flooded with such jelly material and at least to some extent the interstices thereof are filled so as to provide the desired waterproofing effect.
After the application of such jelly, a thermal barrier tape, referred to as a core wrap, and a metal sheathing are then wrapped about the cable core and a jacketing layer of plastic insulation extruded over the sheathing to complete the structure of the cable. Accordingly the jelly application step is one of a series of steps required to form cable of the type under consideration. Such steps are generally conducted continuously and accordingly involve substantial space and equipment outlays. Naturally these additional protective layers further prevent the penetration of moisture to the interior of the cable core. However, as it is possible that such outer protective layers could be broken as by digging operations or earth shift when buried, or accidental blows to such cable when located above ground, it is particularly desirable that all of the voids, even in the inner locations of such core, be completely filled with such waterproofing jelly material.
However, in some applications, particularly those in which several hundred individual insulated strands are utilized to form cable cores, it is extremely difficult in some cases and especially at high production speeds to assure that the proper penetration into the interior interstices of the core by such jelly material is achieved.